Yes, I think it is possible. I suspect unlikely, although I stand to be corrected (as many times in the past.....)

Most machines of this type have a single solenoid operated valve which directs water either through the heat exchanger or into the boiler - it is in effect a diverter - water goes one way or the other.

It would have to jam halfway between the two settings to give this sort of symptom. I don't know enough about them to advise whether this is likely or not. There are bound to be be very knowledgeable folk on here who do though.

It would also mean I think, that water would flow through the grouphead every time the boiler refill was operated. I'm assuming a "jam" - I guess it might be possible for it to operate only half way, but my limited experience of this type of engineering lends me to believe that would be unusual, and that these type of things normally fail either stuck one way or the other.

Ask me about my gas combination central heating and hot water boiler where that happened, and refused to send circulation water though the hot water heat exchanger matrix. result ? cold bath water .....;o(

The valve must be testable. That might be tricky to check it works perfectly, but you probably could establish whether or not it has jammed. Alternatively, it could substituted with a known good valve I guess, or possibly eliminate the valve from the circuit temporarily (with the boiler nice and full to start with).

Pretty sure Isomac will still have spares for this even though it isn't in their product line any more, so if you can pin down the problem, either a boiler/HX unit or a diverted valve is sure to be available as a spare part.

Here is another scenario,yes the Hex can be cracked but is the last possibility, possible but not that common

the water circuit after the pump "tees" one side goes to solenoid 2 way valve to fill the boilerother side goes toone way valve, bypass and the heating exchanger gigleur grouphead etc etc

so if the one way valve is obstructed by scale and doesn't open all the way of completely and the solenoid valve is weak the water will find the less resistance path and it will possibly go in the boiler rather then the H eX

I've pressur tested the hx and the solenoid valve supply to the boiler and both seem fine. So a complete dead end for me I'm afraid.

I embarked on this self diagnosis as Isomac in France and Italy both refused to support the machine without posting it back to them. Not a single repairer in our area and no call out support. We'll be looking for another make that is supported here.

Sorry, we never found a solution or a repairer. Isomac and their representatives have been rudely unhelpful. Great while it was working but scrap at the first fault. Still have an expensive box of bits.

I think I have good news for you. As Stefano suspected, it's the solenoid valve's fault. HX tested for fine and as next step I disassembled the valve wich was stuck (in open position) by scale. The parts are now taking a citric acid bath. Have shoot lot of pictures, If you want to repair yours, I think you can fix it in 2-3 hours (no spare parts needed).

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